修复modal标题属性首次如果没有值，再赋值的话就会没用

examples/routers/modal.vue

@@ -1,12 +1,14 @@
 <template>
     <div>
         <Button type="primary" @click="modal1 = true">显示对话框</Button>
+        
         <Modal
                 v-model="modal1"
-                title="普通的Modal对话框标题"
+                :title="title"
                 @on-ok="ok"
+                :mask-closable="false"
                 @on-cancel="cancel">
-            <p>对话框内容</p>
+            <p><Button type="ghost" @click="title = '这是标题'">设置标题</Button> {{title}}</p>
             <p>对话框内容</p>
             <p>对话框内容</p>
         </Modal>
@@ -16,7 +18,8 @@
     export default {
         data () {
             return {
-                modal1: false
+                modal1: true,
+                title:''
             }
         },
         methods: {

src/components/modal/modal.vue

@@ -261,6 +261,11 @@
                 } else {
                     this.removeScrollEffect();
                 }
+            },
+            title (val) {
+                if (this.$slots.header === undefined) {
+                    this.showHead = val?true:false;
+                }
             }
         }
     };